1. Field of the Invention
The present invention relates to a direct positive type light sensitive silver halide photographic material and a colour diffusion transfer type light sensitive photographic material. Particularly, the present invention relates to such photographic materials, in which a direct positive type light sensitive silver halide emulsion is used which is capable of forming a positive image having a high maximum density and a low minimum density.
2. Description of the Prior Art
There have hitherto been known various direct positive type light sensitive silver halide photographic materials. In a case of a photographic material in which an unfogged internal latent image type silver halide emulsion is used, a positive image is formed by carrying out surface development after or during the fogging treatment following image exposure. In such a case, the fogging treatment may be carried out by a suitable method selected from a method of exposing the entire surface, a method of using a foggant, a method of using a strong developing solution and a method by means of heat treatment.
These techniques are known from various literatures, for instance, from the disclosures in U.S. Pat. Nos. 2,592,250, 2,456,953, 2,497,875 and 2,588,982, U.K. Pat. No. 1,151,363, Japanese Patent Publication No. 27405/68, Japanese Laid-Open Patent Application Nos. 9677/72, 32813/72, 9727/73 and 9717/73, Japanese Patent Publication No. 34213/77, Japanese Laid-Open Patent Application Nos. 8524/75 and 38525/75, and U.S. Pat. Nos. 3,761,266 and 3,796,557.
However, none of these known techniques is capable of providing wholly satisfactory properties required for various photographic applications, such as adequate sensitivity, or sufficient maximum and minimum densities, and there still remain lots of rooms for improvement.
Namely, it is known to carry out the development under a severe condition in order to increase the maximum density of an image. However, this method has drawbacks such that the minimum density is also increased, thus leading to degradation of the sharpness of the image, and that it tends to bring about a degradation of the sensitivity. Further, it is also possible to increase the maximum density by increasing the amount of the silver halide emulsion coated. However, the minimum density is thereby likewise increased, and besides, a greater amount of a silver resource is thereby consumed. Accordingly, this method can not be regarded as a desirable technique also from the standpoint of the present social requirement for conservation of resources.
In view of the application to the field of colour photographs which have become very popular in recent years, an image having a low maximum density and a high minimum density has a fatal defect in that it is thereby impossible to reproduce a beautiful colour photograph. Accordingly, it is an important subject to develop a technique which is capable of overcoming such a defect.